The increased use of polymeric materials, particularly in the building industry, has resulted in increased interest in rendering these materials fire retardant or flameproof. Presently, most commercially available plastics do not possess satisfactory fire retardancy and this inadequacy represents one of the major obstacles to the use of these materials.
The most widely accepted fire retardant chemicals now in use with polymeric materials are antimony trioxide and organohalogen compounds. Of the organohalogen compounds, the best known are chlorendic anhydride (1,4,5,6,7,7-hexachlorobicyclo[2.2.1]-hept-5-ene-2,3-dicarboxylic anhydride); tetrabromo- or tetrachlorophthalic acid; 1,4'-isopropylidenebis(2,6-dichlorophenol) [tetrachlorobisphenol A] or the corresponding bromine-containing compound; Cloran (trademark), i.e., 2,3-dicarboxyl-5,8-endomethylene-5,6,7,8,9,9-hexachloro-1,2,3,4,4a,5,8,8a- octahydronaphthalene anhydride; chlorinated paraffins; and dechlorane (dihexachlorocyclopentadiene).
These organohalogen compounds have only limited utility in polymer compositions due to a number of disadvantages. For example, when such halogen compounds are incorporated into a polymer, various physical properties of the polymer are modified, e.g., change in melt viscosity, which requires higher processing temperatures, decrease in light stability, decrease in thermal stability, increase in density, adverse effects on heat distortion temperature, etc.
Some of these disadvantages have been overcome by the use of halogen-containing polymers as the flame retardant additive. Typical of such polymers are 2-chloropolybutadiene, polyvinylchloride, chlorinated polyethylene and chlorosulfonated polyethylene. However, serious disadvantages are also associated with the use of such polymers. Among these disadvantages are: (1) large amounts of halogen-containing polymers are required in order to obtain satisfactory fire retardancy due to the relatively low halogen content thereof; (2) the halogen-containing polymers have low thermal stabilities; and (3) the blending of the halogen-containing polymer with the polymer to be rendered flame retardant usually requires expensive processing techniques.
Some prior art patents are noteworthy for their disclosures of organohalogen compounds proposed for use as flame retardants for polymeric compositions. For example, U.S. Pat. No. 3,403,036 describes a flame retardant having the general formula: ##STR1## wherein X is selected from the group consisting of bromine, chlorine and fluorine; Y is selected from the group consisting of bromine, chlorine, fluorine, alkyl and alkoxy; and Z is a tetravalent cyclic hydrocarbon radical containing at least five carbon atoms. In each case specified in this patent, each reactant forming the Z radical is characterized as being a cyclic hydrocarbon having two centers of unsaturation. In contrast, the novel compounds of this invention have an oxo bridge between the two centers.
Belgian Patent No. 754,358 discloses the use of furan adducts as flame retardants for polymeric compositions wherein the furan adducts have the general formula: ##STR2## wherein x is halogen, such as fluorine, chlorine or bromine; Y is fluorine, chlorine or bromine or an alkyl, alkoxy, haloalkyl or haloalkoxy, each having 1-10 carbon atoms; and R is hydrogen or an alkyl having 1-6 carbon atoms. In this patent, the furan is used as a dienophile, whereas in the present invention, the furan is used first as a diene and then as a dienophile.
These prior art compounds have been discussed in some detail in order to emphasize the structural differences between these prior art compounds and those of this invention as will become more apparent from the ensuing description of the invention.